Conventional electromagnetic motors generally require provisions for removal of heat. Heat is produced in electric motors and generators by electrical resistance and the sliding friction of slip rings, brushes, and commutators that transmit power between fixed and rotating structures. The conduction of even moderate currents through sliding contacts repeatedly welds and breaks the contacts, causing a continual rearrangement of conducting material. As a result, contact surfaces become rougher with continued use. Brushes, which have a relatively small contact surface area, generally wear out faster than rings. These characteristics of resistive heating, contact welding, and short lifetime of motor parts make conventional electric motors unsatisfactory in some applications and environments. Thus, there is a need for motors having improvements in weight, efficiency, and reliability that are important considerations for applications in hostile environments, such as outer space.
The limitations of electric motors in hostile environments have led to the investigation of alternative types of transducers, actuators, and motors. Piezoelectric actuators, for example, comprising dimorphs of two piezoelectric layers have been described in the following copending U.S. patent applications, which are incorporated herein by reference: Ser. No. 252,197 filed Sept. 30, 1988, and Ser. No. 512,286 filed Apr. 20, 1990. A dimorph is a piezoelectric cell having two layers of piezoelectric shear material with a common central electrode and outer ground electrodes. With the bottom ground electrode fixed to a base, an electric potential applied to the common electrode causes translation of the top ground electrode with respect to the bottom electrode. This piezoelectric translation vector, or stroke, is determined by the direction and strength of the polarizations of the piezoelectric layers, the thicknesses of the piezoelectric layers, and the magnitude and polarity of the applied electric potential. A desired stroke vector can be obtained by selecting an appropriate combination of piezoelectric materials, polarization vectors, layer thicknesses, and electric potential. Because the dimorph has outer ground electrodes, a plurality of dimorphs may be stacked without regard to the electrical state of adjacent material. Piezoelectric shear activation allows bipolar electric drive that doubles the mechanical stroke relative to the stroke available from thickness mode or extension mode activation. Furthermore, shear deformation is 2-dimensional, which allows piezoelectric dimorphs to be affixed to non-deforming surfaces without incurring internal stresses.
Piezoelectric actuators can be constructed of stacked segments to provide a three-dimensional mechanical output. By combining cyclically alternating traction strokes of two or more actuators, walking motion can be produced. "Smooth walking" actuator motion requires non-sinusoidal stroke wave forms to match actuator speed to the surface speed of the positioned object. An electrical system for driving smooth walking actuators is disclosed in copending U.S. application Ser. No. 488,548 filed Mar. 5, 1990, which is incorporated herein by reference. In such a system, each segment of an actuator is connected to an electrical controller by a separate electrical loop. The electrical controller provides a separate resonant electrical signal on each loop to stimulate each segment of the transducer. Each segment reacts to the resonant electrical stimulation on its loop. In general, changes in cosine amplitudes control force while changes in sine amplitudes control speed. Because the segments are coupled together, the overall output of the actuator comprises the vector sum (neglecting coupling effects) of the output reactions of the individual segments of the actuator. Thus, each actuator loop is stimulated electrically by a unique combination of frequency, amplitude, phase, and polarity determined by Fourier mechanical summing to produce a stroke having a predetermined, generally non-sinusoidal wave form.